This invention relates to a semiconductor device which is composed of a heterostructure of dissimilar materials.
Development of a laser diode (LD) or light-emitting diode (LED) emitting the visible light, especially around 400 nm, is presently desired, and recent attention is directed to the II-IV group compounds such as ZnSe and ZnS possessing a band gap of 2.6 eV or more. In these materials, however, it is extremely difficult to form a p-n junction which is necessary for fabricating a diode.
To solve this problem, a second harmonic generator (SHG) using them is now being given considerable attention. Generally, the Group II-VI compound semiconductors exhibitor strong nonlinear optical effect, and have an absorption edge in the short wavelength region, so that they are of outstanding promise as a material for SHG emitting laser light of blue color or an even shorter wavelength. Since this SHG device is not composed of a p-n junction which was absolutely required in the conventional LD fabrication, it is possible to easily obtain an SHG device consisting of Group II-VI compounds.
FIG. 1 shows a structural sectional view of an SHG device using the conventional Group II-VI compound semiconductor the inventors had proposed. Numeral 11 is a GaAs substrate, 12 and 12' are Zns films which are cladding layers for confining light, and 13 is a ZnS.sub.0.5 Se.sub.0.5 film which is a waveguiding layer. The film thickness of the waveguiding layer 13 is 0.5 .mu.m, and that of cladding layers 12, 12' are 2 .mu.m. The optical waveguide length is 15 mm. As the laser light source, GaAs/AlGaAs DH laser is used, and laser light with wavelength of .lambda.=0.83 .mu.m is coupled from one end of the waveguiding layer 13, and is propagated through the optical waveguide. As the laser light propagates through the waveguiding layer 13, second harmonic waves are generated by the nonlinear optical effect of Zn.sub.0.5 Se.sub.0.5, and a laser light of .lambda.=0.42 .mu.m is delivered from the end of the opposite side.
Generally, when forming an optical waveguide, a heterostructure composed of a waveguiding layer and cladding layers having a lower refractive index than the waveguiding layer is employed, and it is possible to confine the light within the waveguiding layer. However, to form an optical waveguide from Group II-VI compound semiconductors, especially to form an optical waveguide for passing light around 400 nm, the materials are limited, and Group II-VI materials different in lattice constant must be grown sequentially. In particular, in the Group II-VI compound semiconductor hetero-junction of the prior art stated above, the lattice mismatch may amount even to several percent, which may seriously affect the characteristics of the device using such materials.
When multi-layers composed of dissimilar materials differing in lattice constant are grown, defects such as misfit dislocation occur due to the lattice mismatch, and they may proliferate to lower the crystallinity. Furthermore, by these defects, diffusion of impurities into the crystals is promoted, and crystals of high purity may not be obtained. Accordingly, absorption of the light to be guided occurs, which may lead to increase of the loss of light propagation. Still further, by the lattice mismatch, the morphology of the hetero-interface or surface is worsened, and the scattering loss of waveguiding layer increases.
Incidentally, as a combination for lattice matching with a GaAs substrate, ZnSSe and ZnCdS are known, however ZnCdS has a wurtzite crystal structure which is undesirable in a high quality ZnS crystal.
In the prior art explained herein, GaAs was used as the substrate, but at the present, the GaAs substrate is very expensive, and Ga and As are very toxic. Nevertheless, many problems are left unsolved for epitaxial growth of Group II-VI compound semiconductor of high quality on an inexpensive and less toxic Si substrate.
Generally, since Si is not a compound, its binding power with a compound having a polarity in crystal structure is weak, and when a compound is directly grown on a Si substrate by epitaxial growth, the adhesion of the growth layer on the substrate is very poor. On the other hand, when a compound is formed on an SiO.sub.2 substrate by epitaxial growth, the adhesion to the substrate is very good, but the obtained growth layer is polycrystalline.
It was hence impossible to grow Group II-VI compounds of sufficiently high quality so as to be applied in a device on a Si substrate by epitaxial growth.